1. Field of the Invention
This invention relates to a process for recovering oil from a subterranean, viscous oil-containing formation. More particularly, this invention relates to a thermal method of recovering oil from a viscous oil-containing formation, especially a highly viscous tar sand deposit, employing a sequence of manipulative steps with steam and hot water to obtain maximum heat utilization and oil recovery.
2. Background of the Invention
Increasing worldwide demand for petroleum products, combined with continuously increasing prices for petroleum and products recovered therefrom, has prompted a renewed interest in the sources of hydrocarbons which are less accessible than crude oil of the Middle East and other countries. One of the largest deposits of such sources of hydrocarbons comprises tar sands and oil shale deposits found in Northern Alberta, Canada, and in the Midwest and Western states of the United States. While the estimated deposits of hydrocarbons contained in tar sands are enormous (e.g., the estimated total of the deposits in Alberta, Canada is 250 billion barrels of synthetic crude equivalent), only a small proportion of such deposits can be recovered by currently available mining technologies (e.g., by strip mining). For example, in 1974 it was estimated that not more than about 10% of the then estimated 250 billion barrels of synthetic crude equivalent of deposits in Alberta, Canada was recoverable by the then available mining technologies. (See SYNTHETIC FUELS, March 1947, pages 3-1 through 3-14). The remaining about 90% of the deposits must be recovered by various in-situ techniques such as electrical resistance heating, steam injection and in-situ forward and reverse combustion.
Of the aforementioned in-situ recovery methods, steam flooding has been a widely-applied method for heavy oil recovery. Problems arise, however, when one attempts to apply the process to heavy oil reservoirs with very low transmissibility such as tar sand deposits. In such cases, because of the unfavorable mobility ratio, steam channelling and gravity override often result in early steam breakthrough and leave a large portion of the reservoir unswept. The key to a successful steam flooding lies in striking a good balance between the rate of displacement and the rate of heat transfer which lowers the oil viscosity to a more favorable mobility ratio. Accordingly, this invention provides an improved thermal system for effectively recovering oil from subterranean formations such as tar sand deposits.